D. P. Malta

Single‐crystal diamond plate liftoff achieved by ion implantation and subsequent annealing

We describe a new method for removing thin, large area sheets of diamond from bulk or homoepitaxial diamond crystals. This method consists of an ion implantation step, followed by a selective etching procedure. High energy (4–5 MeV) implantation of carbon or oxygen ions creates a well‐defined layer of damaged diamond that is buried at a controlled depth below the surface. For C implantations, this layer is graphitized by annealing in vacuum, and then etched in either an acid solution, or by heating at 550–600 °C in oxygen. This process successfully lifts off the diamond plate above the graphite layer. For O implantations of a suitable dose (3×1017 cm−2 or greater), the liftoff is achieved by annealing in vacuum or flowing oxygen. In this case, the O required for etching of the graphitic layer is also supplied internally by the implantation. This liftoff method, combined with well‐established homoepitaxial growth processes, has considerable potential for the fabrication of large area single crystalline dia...

Low‐defect‐density germanium on silicon obtained by a novel growth phenomenon

Heteroepitaxial Ge on Si has been grown using molecular beam epitaxy at a Si substrate temperature of 900 °C. Electron microscopy results reveal a highly faceted interface, indicating localized Ge melting and subsequent local alloying with Si. Furthermore, this phenomenon is associated with extensive threading dislocation confinement near the Ge/Si interface. Etch pit density measurements obtained on Ge heteroepitaxial films that have undergone interfacial melting are as low as 105 cm−2.

SECONDARY ELECTRON EMISSION ENHANCEMENT AND DEFECT CONTRAST FROM DIAMOND FOLLOWING EXPOSURE TO ATOMIC HYDROGEN

Polished nominal (100) surfaces of four types of diamonds were exposed to atomic hydrogen by hot filament cracking of H2 gas or by immersion in a H2 plasma discharge. Both types IIa and IIb (100) diamond surfaces exhibited the following characteristic changes: (a) secondary electron (SE) yield increased by a factor of ∼30 as measured in a scanning electron microscope (SEM), (b) near‐surface, nontopographical defects were observable directly using the conventional SE mode of the SEM, (c) surface conductance increased by up to 10 orders of magnitude. These changes were observed only weakly in nitrogen‐containing types Ia and Ib diamonds.

Method of fabricating a free‐standing diamond single crystal using growth from the vapor phase

By combining a low temperature (600 °C) chemical vapor deposition process for homoepitaxial diamond and conventional ion implantation, we have made and lifted off a synthetic diamond single crystal plate. Before growth, a type Ia C(100) crystal was exposed to a self implant of 190 keV energy and dose of 1×1016 cm−2. Homoepitaxial diamond growth conditions were used that are based on water‐alcohol source chemistries. To achieve layer separation (‘‘lift‐off’’), samples were annealed to a temperature sufficient to graphitize the buried implant‐damaged region. Contactless electrochemical etching was found to remove the graphite, and a transparent synthetic (100) single crystal diamond plate of 17.5 μm thickness was lifted off. This free‐standing diamond single crystal plate was characterized and found to be comparable to homoepitaxial films grown on unimplanted single crystal diamond.

Demonstration of a method to fabricate a large-area diamond single crystal

A multi-step process to fabricate a diamond single crystal that is larger than the original, natural, commercially-obtained crystals is described. Starting with 3.0 mm × 3.0 mm × 0.25 mm, natural, type Ia C(100) crystals that have had their edges oriented to (010) and (001), we have successfully bonded two to a Si substrate in close proximity to each other. Subsequent diamond homoepitaxy using plasma-enhanced chemical vapor deposition of up to

Influence Of Surface Terminating Species On Electron Emission From Diamond Surfaces

75 μm thickness has enabled epitaxial overgrowth to join the two diamonds. The topography was excellent, and microRaman spectroscopy indicated only a 0.6 cm−1 line broadening (crystal degradation) at the joint. The creation of etch pits (via oxidizing flame) on the joined diamond surface indicated a higher defect density at the joint, but this more-defective region was constrained to within the dimensions of the original gap between the diamond crystals. These results indicate that this process of epitaxial joining of diamond single crystals has the potential to be scaled up to larger area in order to fabricate a diamond single crystal of desired area and reasonable crystal perfection.

Etch-delineation of defects in diamond by exposure to an oxidizing flame

Changes in electron affinity on the C(001) surface of type Ifb diamonds have been studied using a variety of surface analytical techniques, including ultraviolet photoemission spectroscopy, secondary electron emission spectroscopy and constant initial states photoemission. Following H-plasma exposure, an intense low-energy emission peak was observed with all spectroscopies. The emission intensity associated with the chemisorbed hydrogen was found to be a linear function of surface hydrogen coverage. The proposed mechanism for the hydrogen induced changes in electron affinity is the creation of a dipole on the surface by the addition of hydrogen which opposes the surface potential of the bare surface. A total change in electron affinity of 2.2 eV was measured upon hydrogen termination of the clean 2x1 surface. Constant initial states photoemission demonstrates that the intense low-energy electron emission observed arises from electrons emitted from bulk states at the conduction band edge. Oxygen, as an electronegative species, was found to have the opposite effect and the electron affinity was increased by ∼3.7 eV upon oxygen termination relative to the clean 2x I surface.

Growth and Characterization of Heteroepitaxial Nickel Films on Diamond Substrates

An experimental study of the etching properties of defects in diamond using propane flame exposure in air is presented. Both natural diamond crystals and polycrystalline diamond films were exposed to a flame for an optimum time of 3 ‐ 4 s. This process topographically delineates defects in diamond via an accelerated etch rate at defect sites. Using transmission electron microscopy (TEM) to determine the exact nature and density of defects present in the diamond, we have found a direct correlation between topographical delineation observed by scanning electron microscopy (SEM) and the defect structure observed by TEM.

Electronic Structure of Polycrystalline PECVD Diamond Surfaces

In the present study epitaxial Ni(001) films have been grown on natural C(001) substrates (type la and Ha) and homoepitaxial C(001) films. Two deposition techniques including electron-beam evaporation of Ni in a molecular beam epitaxy (MBE) system and evaporation of Ni from a resistively heated tungsten filament have been employed. As evidenced by scanning electron microscopy (SEM), the Ni films deposited by electron-beam evaporation were found to replicate the very fine, unidirectional scratches present on the as polished C(001) substrates. Indeed, the coverage and uniformity of the deposited films would imply a two-dimensional (2-D) growth mode. In comparison, the thermal evaporation of Ni on C(001) substrates results in a highly textured and faceted surface morphology indicative of three-dimensional (3–D) nucleation and growth. Moreover, Rutherford backscattering/channeling measurements have demonstrated that the Ni(001) films deposited by electron-beam evaporation are of superior crystalline quality. Differences in the observed microstructure and apparent growth modes of the epitaxial Ni(001) films have been attributed to the presence of oxygen incorporation in those layers deposited by thermal evaporation.

Dense Nucleation of Polycrystalline Diamond Films Using CF4-H2 Low Pressure rf Discharges

Ultraviolet and X-ray photoelectron spectroscopy techniques have been employed in a preliminary study of the electronic structure of polycrystalline diamond films that have been grown on Si substrates by if-plasma enhanced chemical vapor deposition using water/ethanol growth chemistries. In particular, polycrystalline diamond films with distinctly different surface morphologies and Raman scattering characteristics have been investigated. Corresponding ultraviolet photoemission spectra from air-exposed samples have shown the presence of a prominent low-energy secondary electron emission peak indicative of a negative electron affinity (NEA) surface. Chemical stability of the polycrystalline diamond NEA surface has been demonstrated following conventional acid cleans and hydrogen plasma processing. In contrast, an oxygen (20%)/Ar plasma exposure has been shown to extinguish the photoemission of low-energy secondary electrons and remove the NEA. However, by employing a high-temperature anneal at 750 °C for 15 min in ultra-high vacuum the NEA surface can be restored. Compared to NEA single crystal diamond surfaces the photoexcited low-energy electron emission from chemical vapor deposited polycrystalline diamond films is more robust.